Case Study: Use THERM and WUFI-ORNL/IBP to Predict Condensation
and Moisture Content in Wall Assemblies

By Philip Luo, Architect, LEED AP
January 4, 2010

1.0 Introduction

Since the toxic mold litigation case of Ballard vs. Fire Insurance Exchange
in 2001, Architects and building owners have been increasingly concerned
about the liability caused by the presence of mold on occupant health
and indoor air quality. The Ballard case shows that juries were willing
to award multimillion dollar judgments against insurance companies for
liability involving mold contamination.1 Frequently
involved as defendants in mold contamination lawsuits, Architects are
beginning to question if the 'old "rules of thumb" about designing
for moisture control in the building envelope' may be contributing to
the moisture building-up in some buildings2.

Fortunately, there are a number of software applications that can help
Architects evaluate the effectiveness of their envelope design. This article
will investigate two free envelope analysis programs: THERM and WUFI.
THERM is a free program provided by the Lawrence Berkeley National Laboratory
to analyze two dimensional heat transfer through building products. WUFI-ORNL/IBP,
a joint development between the Oak Ridge National Laboratory and the
Fraunhofer Institute, is a hygrothermal model that predicts moisture transport
in building envelope systems over a period of time.

2.0 Rainscreen vs. Metal Panel Wall

The Ventilated Rainscreen is a cladding system that Architects and product
manufacturers have embraced in their effort to improve the moisture performance
of traditional metal panel cladding systems. This study will use THERM
and WUFI to compare the performance of the Rainscreen system against the
traditional metal panel system.

FIGURE 1. TRADITIONAL METAL PANEL SECTION

The traditional metal panel system is mechanically fastened to a metal
stud wall. Between the metal panel and the building enclosure is a layer
of air permeable waterproofing material such as building paper (asphalt
impregnated paper) or building wrap. The stud cavity is insulated with
batt insulation (mineral fiber). Between the metal studs and the interior
gypsum board is the vapor barrier. The vapor barrier keeps warm, moist
air from entering the wall cavity.

FIGURE 2. VENTILATED RAINSCREEN SECTION

The ventilated Rainscreen separates the exterior metal panel from the
building enclosure with a ventilated air space and a layer of rigid insulation.
Instead of allowing air to pass through the waterproofing layer, the waterproofing
layer is also an air barrier as well. The stud cavity is uninsulated and
is not sealed with a vapor barrier. Therefore, air from the building interior
can dry out the stud cavity.

3.0 Cold Climate Thermal Analysis (THERM)

This study uses LBNL THERM software3 to compare the thermal performance
of Metal Panel assembly and the Ventilated Rainscreen assembly in a cold,
urban climate such as St. Louis, Missouri. The 99% Winter Design Condition
data from the St. Louis Lambert International Airport shows air temperature
of 6 °F (-14.5 °C) and Dew Point of -6.5 °F (-21.4 °C).
The indoor temperature is set for 68 °F (20 °C) with 50% Relative
Humidity (RH).

FIGURE 3. THERM METAL PANEL HEAT TRANSFER DIAGRAM

Figure 3 is a THERM color infrared diagram of the temperature model across
the metal panel section. The color diagram shows that the most dramatic
temperature difference occurs in the batt insulation, where the temperature
drops from 58 °F to 10.3 °F from the room side surface to the
outside surface. Any moist air that leaks through a penetration in the
vapor barrier will probably condense when it hits the cold outside surface.
The thermal analysis suggests that there is a great risk of moisture build
up in the wall cavity of the traditional metal panel assembly.

FIGURE 4. THERM RAINSCREEN HEAT TRANSFER DIAGRAM

Figure 4 is a THERM color infrared diagram of the heat transfer model
of a Ventilated Rainscreen assembly. The major temperature change occurs
in the rigid insulation outside the building enclosure. Heat from the
room is able to warm up the stud cavity above the dew point. The thermal
transfer model suggests that there is low risk of condensation.

TABLE 1. DEW POINT ANALYSIS

METAL PANEL

RAINSCREEN

Outdoor Temperature

6 °F

6 °F

Indoor Temperature

68 °F

68 °F

Indoor Relative Humidity

50%

50%

Indoor Dew Point

48 °F

48 °F

Indoor Surface Temperature

62.8 °F

54.1 °F

CONDENSATION_RISK

LOW

LOW

Cavity Air Temperature

38 °F

47 °F

Cavity Dew Point

20 °F

29 °F

Cavity Surface Temperature

10.3 °F

40.6 °F

CONDENSATION RISK

HIGH!

LOW

The dew point analysis in Table 1 illustrates how heat transfer analysis
can be used to determine the risk of moisture. THERM predicts the temperature
across the various components of an assembly; however, it does not model
the moisture content. The user has to draw on other resources to predict
the risk of condensation. I used an online Dew Point Calculator4
to find the dew point in the wall cavity.

4.0 Cold Climate Moisture Analysis (WUFI)

WUFI-ORNL/IBP5 can calculate the thermal and
moisture transfer within an assembly over a period of time. This study
compares the Metal Panel and Rainscreen assembly in St. Louis, MO from
September 22, 2008 through February 1, 2009 (Winter). WUFI's interface
includes an animated chart that tracks changes in the following data over
the time period: temperature (RED), relative humidity (GREEN), and water
content (BLUE). The user can see if and when relative humidity reaches
100% and condensation starts to collect as water content in building components.

FIGURE 5. WUFI METAL PANEL MOISTURE TRANSFER CALCULATION

Figure 5 shows that relative humidity (GREEN) in the Metal Panel stud
cavity reaches 100% (condensation occurs) during the calculation run period.
Additionally, the water content (BLUE) spiking up in the Plywood substrate
confirms the presence of water in the stud cavity. The calculation results
are animated so the user can see condensation in the wall cavity beginning
in December and ending in February.

FIGURE 6. WUFI RAINSCREEN MOISTURE TRANSFER CALCULATION

The relative humidity in Figure 6 Rainscreen Calculation remains within
the normal range (20%-80%) throughout the run period. There is no significant
water content increase in the assembly. The calculation results suggests
low risk of moisture build-up in the Ventilated Rainscreen assembly.

5.0 Conclusion

WUFI tackles the problem of condensation and moisture build-up more directly
than THERM. It predicts when condensation will occur and how much moisture
will be in an assembly over a period of time. The main drawback of WUFI-ORNL/IBP
is the limited library of building materials and the lack of options in
building material thickness. For example, insulation comes in thicknesses
of .089m and .140m. The user cannot build up insulation in 1 inch (.025m)
increments. The free version does not allow the user to edit or add to
the materials library.

THERM is less sophisticated than WUFI but is more flexible. The user
can draw the assembly in question and model it in THERM. Also, THERM can
be used to calculate heat transfer at windows.

Overall, this author was able to reach the same results using THERM and
WUFI. They both predicted low risk of condensation in the Ventilated Rainscreen
and high risk of condensation in the Traditional Metal Panel. If the user
does not have any real life experience to validate the results of either
program, it does not hurt to use one programs to validate the results
of the other.